The present invention relates to a positive charging color toner which is used in image forming apparatuses utilizing an electrophotographic process, such as electrostatic copying machine, laser beam printer and the like. The present invention includes a positive charging one-component non-magnetic color toner.
In image forming apparatuses utilizing a electrophotographic process, a more simple one-component toner, particularly a non-magnetic one-component toner, has recently been developed in place of a so-called two-component toner using a magnetic carrier and a toner.
In a developing method using the non-magnetic one-component toner, as shown in FIG. 1, first, a toner 1 is charged by frictional charging between a developing roller 2 and a control blade 3, and then adhered on the surface of the developing roller 2 by an image force effect, thereby to form a thin layer of the charged toner 1.
Then, when this thin layer is contacted with an electrostatic latent image on a photoconductor drum 4, the toner 1 in the thin film is transferred to the photoconductor drum 4 according to a potential of each portion of the electrostatic latent image and the electrostatic latent image is developed into a toner image. That is, a so-called reversal development type contact developing method is employed.
As described above, since the toner 1 is circulated in a developing device 8 while being contacting with the control blade 3 and photoconductor drum 4, high durability (strength), which makes it possible to hardly cause deterioration due to heat or pressure generated by friction with the developing roller 2, is required. It is essential to prevent fusion between toners 1 due to heat or pressure generated by friction, or fusion of the toner 1 to the control blade 3 so as not to lower image formation, particularly.
In the latest image forming apparatus, since the requirements such as high speed and low cost have been increased more and more, there has been studied to develop an image forming apparatus wherein conditions of friction to the toner 1 are more severe, for example, a moderate-speed image forming apparatus, or a recycle type image forming apparatus wherein, after the toner 1 was transferred on the photoconductor drum 4, the toner recovered without being transferred on a paper is utilized by transferring to the developing device 8 again. Excellent durability capable of sufficiently coping in these image forming apparatuses is required to the toner 1.
On the other hand, regarding the above toner image to be formed by the development, it has rapidly been developed to change a conventional monochromic development into a full-color development capable of forming an image by registration of toners of yellow, magenta and cyane. According to the full-color development, a multi-color image can be obtained by subjecting a multi-color original to color separation, exposing the multi-color original to light, repeating this step plural times using the above toners of three colors, followed by registration of toner images.
Accordingly, in the full-color development requiring plural developments and registration of a toner layer having a different color onto the same recording medium as a fixing step, a toner having excellent transparency and color mixing property is desired.
Furthermore, with the increase of requirements such as high speed and long life to the above image forming apparatuses as well as requirements of power saving, the low-temperature fixing temperature is apt to be reduced to respond to the requirements and a toner having good fixing property at a lower temperature is required.
Therefore, a polyester resin, which is superior in these characteristics, has been studied as a fixing resin of the toner.
By the way, as the developing method using the non-magnetic one-component toner, the above-described reversal development type contact developing method used in a plain paper facsimile device, a laser printer or the like is popular.
In such a developing method, it is necessary to use a toner and a photoconductor of the same charging type. However, when charging a negatively charging type photoconductor used in combination with a negatively charging type toner, which is often used at present, a large amount of ozone is evolved from a charger and, therefore, it becomes necessary to take a measure against leakage of this ozone outside, resulting in a complicated device. Furthermore, the photoconductor is deteriorated by ozone, thereby reducing the life of the photoconductor drum 4.
Therefore, in the above reversal development type contact developing method, a combination of a positive charging photoconductor and a positive charging toner is required.
However, since the above polyester resin is a polycondensate of a polybasic acid and a polyhydric alcohol, its acid value becomes higher when a lot of unreacted carboxyl groups are present at the polymer terminal and it becomes difficult to charge positively. It has also been known that the polyester resin is insufficient in environmental resistance because an influence of temperature and humidity is liable to be exerted.
Therefore, there have hitherto been suggested various color toners wherein a polyester resin having a specific acid value and/or a specific hydroxyl value is used as the fixing resin to stable the positively charging property of the toner and to improve the environmental resistance (Japanese Laid-open Patent Publication Nos. 220173/1988 and 220174/1988 and Japanese Examined Patent Publication No. 66201/1995).
However, all of toners using a polyester resin having a predetermined acid value and/or a specific hydroxyl value disclosed in the above publications are insufficient in stable positively charging property. The carboxyl group and hydroxyl group contained in the polyester resin are liable to absorb water because they are hydrophilic. Particularly, preferable uniform charge amount can not be obtained under severe conditions of high-temperature/high-humidity and the image is deteriorated.
It is suggested to contain a positive charging electric charge controlling material such as nigrosine dye, quaternary ammonium salt or polymer having a quaternary ammonium salt group (electric charge controlling resin) in the specific polyester resin disclosed in the above publications in accordance with a commonly used technique, thereby to impart good positively charging property to the toner.
However, the nigrosine dye and quaternary ammonium salt are liable to be thermally decomposed in the kneading process in the production of the toner, and can not impart sufficient positively charging property to the toner. Furthermore, the electric charge controlling resin is liable to cause a problem that the transparency and positively charging property of the toner are lowered if the compatibility with the fixing resin is not sufficient.
Furthermore, all of the above publications do not teach or suggest a means for reducing deterioration of the toner due to friction with the developing roller 2. Therefore, in the toner disclosed in these publications, the durability (strength) to the above-described moderate-speed image forming apparatus has still to be sufficient.
When the polyester resin is used as the fixing resin, good low-temperature fixing property is obtained. On the other hand, since the offset resistance is insufficient, there was a problem of a so-called offset phenomenon, that is, when the surface of a heating roller is contacted with the surface of a toner image by a heat roller fixing manner, a toner is adhered on the heating roller, thereby to transfer the toner to the following transfer paper.
Japanese Laid-open Patent Publication No. 82957/1996 discloses a color toner comprising a polyester resin having an acid value of not more than 5 KOHmg/g and waxes having an acid value of not more than 20 KOHmg/g as a releasing agent (anti-offset agent).
In the toner with the construction of the above publication, by defining the acid value of the polyester resin and waxes, uniform positively charging property is imparted to the toner and, at the same time, the offset resistance is improved.
However, since the wax is contained in the toner, the transparency and positively charging property of the toner are likely to be lowered if the wax is not sufficiently dispersed in the toner.
With the increase of the number of copies, the toner is gradually accumulated and fused on the surface of the photoconductor drum, thereby to cause filming.
Furthermore, all of the above publications do not teach or suggest any means for reducing wear of the photoconductor drum.
That is, it is an object of the present invention to provide a positive charging color toner having excellent positively charging property and environmental resistance as well as high durability without adversely affecting low-temperature fixing property and transparency of a polyester resin.
It is another object of the present invention to provide a positive charging one-component non-magnetic color toner which is superior in transparency and positively charging property, and which can reduce wear of a photoconductor drum and can prevent filming, without adversely affecting low-temperature fixing property of a polyester.